A method for a radioimmunoassay of human renin substrate (angiotensinogen) is proposed. The research proposed involves the selective cleavage of a peptide moiety from the human renin substrate, and the conversion of this peptide into a suitable antigenic substance. The antibody formed to this antigen will be utilized in the conventional methods of radioimmunoassay. By use of this method, plasma renin substrate concentrations in various clinical disorders will be determined and evaluated.